Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395, 5,657,607, and 5,902,652, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1-2 illustrate a conventional vacuum IG unit (vacuum IG unit or VIG unit). Vacuum IG unit 1 includes two spaced apart glass substrates 2 and 3, which enclose an evacuated or low pressure space 6 therebetween. Glass sheets/substrates 2 and 3 are interconnected by peripheral or edge seal of fused solder glass 4 and an array of support pillars or spacers 5.
Pump out tube 8 is hermetically sealed by solder glass 9 to an aperture or hole 10 which passes from an interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. A vacuum is attached to pump out tube 8 so that the interior cavity between substrates 2 and 3 can be evacuated to create a low pressure area or space 6. After evacuation, tube 8 is melted to seal the vacuum. Recess 11 retains sealed tube 8. Optionally, a chemical getter 12 may be included within recess 13.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows. Glass frit in a solution (ultimately to form solder glass edge seal 4) is initially deposited around the periphery of substrate 2. The other substrate 3 is brought down over top of substrate 2 so as to sandwich spacers 5 and the glass frit/solution therebetween. The entire assembly including sheets 2, 3, the spacers, and the seal material is then heated to a temperature of approximately 500° C., at which point the glass frit melts, wets the surfaces of the glass sheets 2, 3, and ultimately forms hermetic peripheral or edge seal 4. This approximately 500° C. temperature is maintained for from about one to eight hours. After formation of the peripheral/edge seal 4 and the seal around tube 8, the assembly is cooled to room temperature. It is noted that column 2 of U.S. Pat. No. 5,664,395 states that a conventional vacuum IG processing temperature is approximately 500° C. for one hour. Inventor Collins of the '395 patent states in “Thermal Outgassing of Vacuum Glazing,” by Lenzen, Turner and Collins, that “the edge seal process is currently quite slow: typically the temperature of the sample is increased at 200° C. per hour, and held for one hour at a constant value ranging from 430° C. and 530° C. depending on the solder glass composition.” After formation of edge seal 4, a vacuum is drawn via the tube to form low pressure space 6.
Unfortunately, the aforesaid high temperatures and long heating times of the entire assembly utilized in the formulation of edge seal 4 are undesirable, especially when it is desired to use a heat strengthened or tempered glass substrate(s) 2, 3 in the vacuum IG unit. As shown in FIGS. 3-4, tempered glass loses temper strength upon exposure to high temperatures as a function of heating time. Moreover, such high processing temperatures may adversely affect certain low-E coating(s) that may be applied to one or both of the glass substrates in certain instances.
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The x-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the y-axis is indicative of the percentage of original temper strength remaining after heat exposure. FIG. 4 is a graph similar to FIG. 3, except that the x-axis in FIG. 4 extends from zero to one hour exponentially.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (° F.). The different curves/lines are 400° F. (across the top of the FIG. 3 graph), 500° F., 600° F., 700° F., 800° F., 900° F., and 950° F. (the bottom curve of the FIG. 3 graph). A temperature of 900° F. is equivalent to approximately 482° C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal 4 in FIGS. 1-2. Thus, attention is drawn to the 900° F. curve in FIG. 3, labeled by reference number 18. As shown, only 20% of the original temper strength remains after one hour at this temperature (900° F. or 482° C.). Such a significant loss (i.e., 80% loss) of temper strength is of course undesirable.
In FIGS. 3-4, it is noted that much better temper strength remains in a thermally tempered sheet when it is heated to a temperature of 800° F. (about 428° C.) for one hour as opposed to 900° F. for one hour. Such a glass sheet retains about 70% of its original temper strength after one hour at 800° F., which is significantly better than the less than 20% when at 900° F. for the same period of time.
Another advantage associated with not heating up the entire unit for too long is that lower temperature pillar materials may then be used. This may or may not be desirable in some instances.
Even when non-tempered glass substrates are used, the high temperatures applied to the entire VIG assembly may melt the glass or introduce stresses. These stresses may increase the likelihood of deformation of the glass and/or breakage.
Lead-free frits sometimes are used when forming edge seals. Although lead-free fits are advantageous for a number of reasons (e.g., from environmental perspectives), the use of lead-free frits as edge seals for VIG units sometime is troublesome. For example, it is believed that all commercially available lead-free frits currently melt at temperatures in the range of 500 degrees C. to 600 degrees C. As is known, these temperatures are close to the softening point of soda lime glass, which may be used as the glass substrates in the VIG unit. Thus, it will be appreciated that the current process of melting lead-free frits typically disadvantageously softens the substrates that ultimately comprise the VIG unit. Additionally, exposing the substrates to such high temperatures typically causes them to lose at least some of the strength provided during heat treatment (HT). For example, tempered soda lime glass substrates sometimes actually may de-temper at these temperatures. Furthermore, the VIG unit manufacturing process typically takes a long time, inasmuch as a long time is required to reach and then cool down from these high temperatures. Accordingly, conventional lead-free frits lead to the some or all of the same or similar problems noted above.
Thus, it will be appreciated that there is a need in the art for a vacuum IG unit, and corresponding method of making the same, where a structurally sound hermetic edge seal may be provided between opposing glass sheets. There also exists a need in the art for a vacuum IG unit including tempered glass sheets, wherein the peripheral seal is formed such that the glass sheets retain more of their original temper strength than with a conventional vacuum IG manufacturing technique where the entire unit is heated in order to form a solder glass edge seal. It also will be appreciated that improvements to the ways in which lead-free frits are made and/or incorporated into VIG units would be desirable.
One aspect of certain example embodiments relates to providing an increased amount of ferrous oxide in the frit as opposed to the glass substrates. Accordingly, one aspect of certain example embodiments relates to providing a fit that has a glass redox (FeO/Fe2O3) greater than the glass redox (FeO/Fe2O3) of the two substrates comprising the VIG unit. In certain example embodiments, the frit's glass redox (FeO/Fe2O3) preferably is at least about 0.02 higher than either (or the higher) of the substrates' glass redox (FeO/Fe2O3), more preferably at least about 0.04 higher than either (or the higher) of the substrates' glass redox (FeO/Fe2O3), and most preferably at least about 0.06 higher than either (or the higher) of the substrates' glass redox (FeO/Fe2O3). This addition advantageously causes more energy from an infrared source to be absorbed by the fit and less energy to be transmitted through the frit. In certain example embodiments, the glass frit may be heated using one or more infrared source(s), e.g., operating at IR wavelengths in the range of 0.9-1.2 microns, for example.
Another aspect of certain example embodiments relates to providing a glass frit for a VIG unit edge seal having a glass redox (FeO/Fe2O3) of preferably 0.20 to 0.30, more preferably 0.21 to 0.28, and most preferably 0.22 to 0.25. Total iron content (expressed herein as Fe2O3) preferably is 0.5% to 5%, more preferably 0.75% to 3%.
Still another aspect of certain example embodiments relates to reducing the time in which the glass fit melts. For example, certain example embodiments may involve the melting of the glass fit at a temperature of 450 degrees C. (or less) after about 10 minutes.
Yet another aspect of certain example embodiments relates to maintaining the heat treatment (HT) strength (e.g., tempering strength) of the substrates of the VIG unit. This may be accomplished in certain example embodiments by heating the glass frit to a higher temperature than the substrates, e.g., over the same or similar amount of time.
Certain example embodiments of this invention relate to a vacuum insulating glass (VIG) intermediate assembly. First and second substantially parallel spaced-apart glass substrates are provided. The first and second substrates each include one or more edge portions to be sealed. A glass frit is provided at least partially between the first and second glass substrates for sealing said one or more edge portions to be sealed. The glass frit has a glass redox (FeO/Fe2O3) that is higher than a glass redox (FeO/Fe2O3) of the first and second substrates.
Certain example embodiments of this invention relate to a glass frit for a vacuum insulating glass (VIG) unit. The glass fit has a glass redox (FeO/Fe2O3) of 0.20 to 0.30 and a total iron content (expressed in terms of Fe2O3) of 0.5% to 5%. The glass frit absorbs infrared energy having a wavelength of 0.9-1.2 microns such that less than 15% of the infrared energy is transmitted through the glass frit. The glass frit absorbs infrared energy such that it reaches a melting temperature in 10 minutes or less. The glass frit melts after exposure to a temperature of 400-450 degrees C.
Certain example embodiments of this invention relate to a method of making a vacuum insulating glass (VIG) unit. First and second substantially parallel spaced-apart glass substrates are provided, with the first and second substrates each including one or more edge portions to be sealed, a glass frit is provided at least partially between the first and second glass substrates for sealing the one or more edge portions to be sealed. Infrared energy is irradiated from one or more infrared energy sources towards the one or more edge portions to be sealed in forming an edge seal of the VIG unit. The glass frit has a glass redox (FeO/Fe2O3) that is higher than a glass redox (FeO/Fe2O3) of the first and second substrates.
Certain example embodiments of this invention relate to a method of making a vacuum insulating glass (VIG) unit. First and second substantially parallel spaced-apart heat treated (HT) glass substrates are provided, with the first and second substrates each including one or more edge portions to be sealed. A glass frit is provided at least partially between the first and second glass substrates for sealing the one or more edge portions to be sealed. Infrared energy is irradiated from one or more infrared energy sources towards the one or more edge portions to be sealed in forming an edge seal of the VIG unit. The glass frit includes an increased amount of ferrous oxide such that the irradiating of the infrared energy causes the first and/or second substrate to reach a first elevated temperature and the glass frit to reach a second elevated temperature, with the second elevated temperature being higher than the first elevated temperature and with the first elevated temperature being sufficiently low to reduce the likelihood of the first and/or second substrate melting and losing HT strength.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.